CA1136259A - Broadband seismic energy source - Google Patents

Abstract

ABSTRACT
A vibratory seismic energy source capable of generating signifi-cant energy over a broad frequency band. The vibrating baseplate and asso-ciated structure are designed for minimum size and weight, while retaining the structural strength to permit high actuator forces. The weight of the transport vehicle is applied to a substantially horizontal hold-down plate that is interposed between the piston-cylinder assembly and the baseplate.
This horizontal plate is coupled to the baseplate through resilient means having compliance such that dynamic motion of the baseplate is decoupled from the hold-down plate and transport vehicle.

Description

This application is a divisional of S~No 284,737 filed lS August 1977 and is directed to a seismic energy source. The parent application and other divisionals S.N. 381,210, S.N. 381l211, S.N. 381,213, S.~. 381,214 and S.N. 381,215, all filed 6 July 1981 are also directed to seiBmic energy sources.
Th$s invention relates to improvements in seismic energy sources, and in particular, to a broad band vibratory seismic energy source.
In the practice of exploration selsmology for the location of subsur~
face petroleum accumulations, it is necessary to provide a source of energy for inducing propagating elastic waves in the area oE the earth to be ex-plored. These elastic waves propagate down into the upper crustal material of the earth, are reflected from impedance d~continuities located therein, and are subsequently detected by geophones or seismometers located at the surface of the earth. The records produced by the geophones or seismometers contain much valuable inf ormaton about the crustal s~ructure of the earth and may be used to ascertain the existence of petroleum accumulations. It has become common in many cases to use, as the source of propaga~ing elastic waves9 a hydraulically operated vibratory source more æimply referred to a~
a vibrator.
In a eypical e~bodiment, a vibrator comprises a double ended piston rigidly ~ffi~ed to a coaxial piston rod. The piston is loc~ted in recipro-cating relationship in a cylinder formed within a heavy reaction mass.
Means are included for alternately introducing hydraulic fluid under high pressure to opposite ends of the cylinder, thereby imparting a reciprocating motion to the piston relative to the reaction mass. The pi~ton rod ex~end-ing from the reaction mass is rigidly coupled to a baseplate ~hich is main-tained in intimate contact with the earth ~aterial. Since the inertia of the reaction mass tends to resist displacement of the reaction mass relative -to the earth, the motion of the piston is coupled through the piston rod and baseplaee to impart vibratory seismic energy in ~he earth.
Typically, the vibrators are transported by truck, and it is also kno~n ~o prevene decoupling of the baseplate from the ground by applying a

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1 portion of the truck's ~eight to the baseplate during operation. The weight of the truck is frequently applied to the baseplate through one or more spring members~ each having a large compliance 9 with the result that a static bias force is imposed on the baseplate, while the dynamic forces of the baseplate are decoupled from the truck itself.
Conventional vibrators are capable of effective operation over a relatively small range of low frequencies, typically 5 to 70 hert~. In the past, these relatively low fre~uency vibrators have proven to be useful seismic energy sources. As existing oll reserves become depleted, however, 1~ it becomes necessary to search deeper and with increased resolution to locate additional reserves. A broad band vibrator (BBV) capable of opera-tion over a band of frequencies ~ider than those previously achievable with known vibrators is useful in providing greater resolution and meaningful interpretation at greater depth. In order to operate the vibrator so as to provide significant ~utput~force levels at high frequencies, it is ~-necessary to minimize the weight of the baseplate and of other structural elements rigidly affixed to the baseplate. In this way, the inertial force hich must be overcome solely t~ move the baseplate weight is minimized.
Further, it is necessary to provide sufficient force acting on the piston to overcome the inertial force of the baseplate structure and still induce significant energy in the earth.
The present inventlon provides various improvements in a BBV.
In one aspect the invention provides a seismic energy source comprising in combination: a) a reaction mass having located therein a cylindrical ~ `
aperture, b) an actuator rod comprising a piston and piston rod, the piston ~ ~-being located in the cylindrical aperture, and c) means for introducing a fluid under high pressure into the cylindrical aperture to exert a force on the piston, d) the actuator rod having an internal bore with a diameter that varies as a function of position along the rod.
In a second aspect the invention provides a vibratory seismic en~rgy source having a double ended piston reciprocably mounted in the cylinder of a reaction mass and a piston rod extending from opposite ends 1 f the piston to project f~o~ the reaction mass and further co~prisi~g a pair of relatively soft bushings lining the bore of the ~eaction mass to .
provide mechanical support for the portions of the piston rod located within the reaction mass and for portions of the piston, the travel of the piston .
within the cylinder being limited so that each end of the piston remains within one of the bushings at all times. ~ :
In another aspect the invention provides a vibratory seismic energy source having a piston reciprocably mounted within the cylinder of a ~ :.
reaction mass and at least one port in the wall of the cylinder for admitting high pressure fluid to exert a force acting on the piston, the piston being disposed in the cylinder such that when a piston over travel condition occurs~ the piston substantially restricts the flow of fl~lid through the at least one port thereby trapping a volume of fl.ui.d between the face of the piston and the cylinder, whereby a braking force is generated. `-~
In still another aspect the invention provides a seismic energy ~`
source comprising in combination: ai force generating means for providing a force to be imparted to the earth and b) an aluminum baseplate for coupling ;~..
the force to the earth, the basepla-te defining a grid of internal cavities, :
each cavity enclosed by ~our plane surfaces parallel to the general pl.ane of :
20 the baseplate and by two plane surfaces parallel to the general plane of the .
baseplate intersecting the four perpendicular planes at the top and bottom thereof respectively. ;

In another aspect the invention provides a vibratory seismic ~ .
energy source comprising: a) a reaction mass having a hydraulic cylinder ~:
formed therein, b) an actuator rod including a piston reciprocably located within the hydraulic cylinder, c) a baseplate, d) a frame member coupled to an end of the actuator rod, and e) a plurality of stilt legs extending from the frame member to the baseplate.
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A further aspect of the in~ention is a vibratory seismic energy source transported by a vehicle and including a baseplate for coupling seismic energy to the ground, at least one structural member adapted to support at least a portion of the weight of the vehicle thereon, means for coupling substantially vertical forces from the structtlral member to the baseplate, and stabilizing means for limiting translation of the structural member relative to the baseplate. The stabilizing means may comprise a -3b- :

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1 center link pivotally coupled to t~e baseplate~ ~ first stabilizing rod ~
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rotatably coupled at one end to a polnt on the center link above the level of its pivot and at the other end to a first point of the structural member, and a second stabilizing rod rotatably coupled at one end to a point on the center link below the level of its pi~ot and at the other end to a second point of the structural member. Alternatively the center link can be pivotally coupled to the structural member and the stabili~ing rods to the baseplate.
Finally, there is another aspect of the invention whicl- is a vibratory seismic energy source adapted to be transported on a vehicle, including a vertically disposed piston and cylinder assembly, and a piston ;~`
rod extending downward from the piston and connected at its lower end to a baseplateS a rigid plate interposed between the piston and cylinder assembly and the baseplate, and adapted to permit motion of the piston rod . ~ .
in relation to the rigid plate, elastic means supporting the rigid plate from the baseplate and means for imposing at least a portion of the weight of the vehicle on the rigid plate. `
Preferred characteristics of the seismic energy source will be discussed in the following description:

The ground contacting surface of the baseplate in the preferred embodiment has an area substantially leæs than that of conventional vibrators in the 20,00Q to 30,000 pound eorce range and has a square shape in contrast with the typical rectangular shape of other vibrators. The baseplate area of the BBV is less than that of existing vibrators, even though the peak actuator force of the latter may be less than half that of the BBV.
The piston rod of the BBV is coaxial with the major axis of the piston, and extends from both sides thereof. The lower segment of the ~ ;
piston rod is rigidly coupled to the center of the baseplate, while the -upper segment of the piston rod is coupled through a structure consisting of four stilts to the four corners of the baseplate. Thus, the baseplate is supported both at its center and a~ each of its Eour corners. ~his type of support, coupled with the 1~nique two dimensional I-beam structure

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~ , 1 of the aluminum baseplate~ provides a very light baseplate structure capable of withstanding the high force levels generatred by the actuator of the BBV.
Most known vibrators are carried by a vehicle having a source of motor power located near the front end of ~he vehicle and a drive line extending therefrom to ~he rear end of the vehicle so as to engage a differential assembly and ultimately to power the rear wheels of the vehicle. Typically, the drive line extends through a por~ion of the vibrator itself, alld it is necessary to configure the vibrator such that there is suE~icient clearance for the drive line, whether the vibrator be in its raised or lowered position. In the case of the BBV, the drive line is ellminated and in the absence of a need to provide clearance for a drive line, the vibrator dimensions, particularly its vertical extent, can be substantially reduced. With the reduced dimensions, the stresses imposed on certain structural members~ particularly the stilt legs, are less severe and it becomes possible to make these members lighter.

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l In view of the reduced di~ensions of the baseplate~ previously 2 known means for imposing the weight of the vehicle on the baseplate itself `
3 become impractical. Accordingly, in an embodiment of the invention, a 4 unique one piece hold-down plate is provided to couple the weight of the vehicle through air bags to the baseplate of the vibra~or. Further in this 6 connection, and again in view of the reduced baseplate dimensions, the use 7 of radius rods as a means for providing la~eral stabillty between the hold-8 down plate and the baseplate becomes difficult. In an embodiment of the 9 inventlon, a system of linkages sometimes referred to as a Watt's linkage ~
1~ and adapted to the reduced baseplate dimensions, provides excellent lateral - ;
11 stability.
12 In the BBV, the reaction mass is a multiple piece struc~ure 13 wherein the plurality of subassemblies are bolted together so as to provide 14 a unitary reaction mass. Thls economical means of fabrication results in a reaction mass which provldes the necessary high mass within the confines 16 of the stilt legs themselves. The stilt legs slant inward as they extend :
17 upward from the baseplate, so as to provide effective resistance to hori- ;

18 zontal stress, as well ag to the vertical stresses generated by the vibra-19 tor itse}f.

The ac~.uator piston and pis~on rod assembly of the BBV is pro-21 vided with a hollow tapered bore, designed to result in constant stress as ;

22 a function of length along the rod and to minlmize the mass of the piston 23 and rod assembly itself. Further9 the piston and piston rod assembly ~;

24 cooperates with the cyllnder of the reaction mass so as to provide an internal braking mechanism for lil~iting piston over-travel.

26 Other objects and features of the invention will become obvious 27 from a consideration of the following detailed description when taken in 28 connection with the accompanying drawings wherein:

29 FIGURE 1 is a perspective view of a BBV. ~ ;~
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FIGURE 2 is a cross sec~ional schematic view of portions of the 31 vibrator.
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1 FIGURE 3 is a perspecti~e view (partially cut away) of a base-2 plate.
3 FIGURE 4 illustrates the vibrator mounted in a ~ruck.
4 FIGURE 5 is a perspective view illustrating stabilizing means for a vibrator.
6 FIGURE 6 illustrates an alternative configuration for the sta~
' 7 bilizing means.
8 FIGURE 7 is a partial sectional view of the cylinder of a prior 9 art vibrator and of the BBV.
FIGURE 8 ls a sectional vlew of a vibrator actuator rod.
11 FIGURE 1 is a perspective view of a vibrator, portions oE which 12 are showll in cross section in FIGURE 2. ReEerring to FIGURES 1 and 2, a 13 baseplate 17 is driven by a hydraulic drive mecha~ism comprising a driving 14 piston 103 reciprocably mounted within a cylindrical bore 101, alld a piston rod 104 carrying piston 103 and extending from both the top and bottom of 16 the cylinder housing lOla. The lower~end of piston rod 104 is rigidly 17 affixed to the center of the baseplate and the top is rigidly affixed to 18 the center of frame member 106. ~Stilt legs 108a, 108b, 108c, and 108d (the 19 latter not visible in FIGURE 1) extend in slanting relationship from the baseplate to join the corners of frame member 106 to the corners of the 21 baseplate.
22 It is an ob~ect in constructing the BBV to reduce the mass moving . .
23 with the ground while the vlbrator is 8enerating a seismic wave. This 24 "baseplate weight" consists of the actual baseplate which couples the actuator force to the ground and all components, structures, and members 26 rigidly attached to the baseplate. A minimum baseplate weight is of 27 paramount importance i~ high frequency operation. With a fixed peak ;~
28 actuator force available, the only force which may be coupled into the 29 ground is the actuator force minus the force used to move the baseplate 30 weight. The less force required to move the baseplate, the more there is ` ;~
31 available to be imparted to the ground.

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1 The equation for the force magnitude required to move a mass at a 2 specific frequency, assuming sinusoidal mo~ion, is: ~
3 F=(W/g)A(2 ~f) sin (2 ~ft) ~ ;
4 where:
S F= force, 6 W= "basepla~e weight", ;~
7 A= mass di~platement (1/2 of peak-to-peak)~
3 g= aeceleratlQn of gra~ity, 9 f~ driving frPquency, and t= time.
11 Wlth a displacement of A, the force required increases with 12 frequency as a function of tha frequency squsred. This illustrates the 13 need for a minimum baseplate weight W for high frequency operation.
14 To achleve the obJective of mlnlmum baseplate weight while re~
15 taining structural rigidity9 the ground coupling plate (baseplate) is ;
16 constructed from two alumlnum sections9 17a and 17b, the internal surfaces 17 of which have a honeyco~b appearance. One surface o each of the plate ~`-18 ;sec~ions may bè milled out as illustrated ln FI&UR~ 3. In a pre~erred 19 embodiment, the baseplate may be square, having dimensions of four feet on ~--20 each sid0. Each oF the baseplate sections may be approximately 3-1/2" ~;
21 thick. A8 shown in FIGURE 3, the internal sides of both sectlons of the 22 bassplate are ~illed out in a 16 x 1.6 grid, o~ 3" each. Indivldual sections 23 may be milled out to a depth of 2.85 inches, with .25 lnches left between 24 each milled out ~ection. I~ should be emphasized that the dimensions can vary, dependlng on the force and rigidi~y requirements of a particular ~6 vibratory structure.
27 Certain portions of the baseplate sections, such as tho~e deslg-28 nated by numeral Z9, are lgt unmilled in order to pr~vide locations for 29 boltlng the two sections together~ In ~he top portion of the baseplate, holes, such as those deslgnated by numeral 31 are drilled9 and bolts, such ~, :

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1 as bolt 32, are inserted through these holes to secure the ~wo sections of 2 the baseplate tog~ther. Gorresponding holes, such as those designated by 3 numeral 33, are drilled into the bottom section oE the baseplate. Sc~ew 4 threaded receptacle means are inserted ln the bottom section of the base-S plate to receive the bolts inserted through the holes in the top section.
6 In general, the holes utllized for securing the two sections of the base-7 plate together are posltioned so that the connecting bolts also affix 8 additional vibrator structure to the baseplate.
9 It is known in the seismic vlbrator art to fabricate the gene-rally rectangular baseplate of a plurallty of parallel steel I-beams. The 11 longltudlnal axes of the I-beaMs are located parallel to the ma~or axis of 12 the rectangle and adJacent I-beams are oriented so the edges of their upper 13 and lower flanges are abutting (the webs of the I-beams lle in vertical 14 planes). The I-beam flanges are welded together so as to provlde a unitary structure which may be further reinforced by top and bottom reinforcing 16 platesO It will be appreciated that this type of structure pro~ides great 17 r~slstance to stress exerted along the major axis of the xec~angle. While I8 the resistance to stress may be expected to be less along the minor axls of 19 the rectangle, the use of steel I~beams resulted ln a baseplate wlth suffi~
20 clent stress tolerance for the force :Levels employed in prior art vibra- ;
21 tors.
22 In the case of the BBV, the use of a steel baseplate structure 23 would impose a severe weight penalty on the vibrating mechanism. Alterna-24 tively, an aluminum baseplate structure comprlsed of a plurality of one-: -::-dimensional I-beams might not have sufficient stress reslstance for tile 26 forces generated by the BBV. It will be appreciated that the two-piece ;~
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27 a]uminum baseplate structure of the present invention, when bolted together 28 as set forth above, comprises a unique two-dimensional I-beam structure. ~

29 As a result of this unique s~ructure, the baseplate may be maintained ;

33 within acceptable weight limitations and yet be capable of wlthstandlng the ' : ~ .' ~ ~L3~

1 large forces generated by the BBV. In this description and in the accom- ~;
2 panying claims the tenn "two~dimensional I-beam structure" is considered to 3 include structure such as that illustrated in FIGURE 3 as well as similar 4 structures such as one having a plurality of I-beams radiating from the baseplate center. Further, in the structure of FIGURE 3, it is not neces-6 sary that the I-beam webs be coplanar from cell to cell of the structure.
7 The center of the baseplate is rigidly connected to the lower end :
8 of piStOII rod 104. Piston rod 104 may have an annular flanged portion at 9 the lower end with a plurality of holes therein. The same bolts that function to secure the center portions of the baseplate sectlons together affix the ;~
11 piston rod to the baseplate. ~ ~
12 As statPd earlier, the top of piston rod 104 is connected to the ~ ~ ;
13 center of frame member 106. Connected at each corner of this frame member 14 is an inclined stilt leg which is fixedly connected to a corner of the baseplate. When vibrations are induced by controlled hydraulic fluid flow 16 into and from cylinder 101, motion ganerated in the piston is transmitted 17 to the center and to each of the four corners of the baseplate. This ~ - ;
18 configuration is a very rigid vibratory structure which produces a uniform 19 movement of the entire baseplate closely corresponding to the motion of the piston.
21 The stilt legs 108 are subiected to a complex form of loading.
22 The loading consists of vertical and horizontal forces. The vertical load~
23 ing ls due to the simple vibratory operational mode of the piston 103. The 24 piston rod 104 is of adequate strength to withstand the vertical loading.
The horizontal loading is due to ground rocking and ground 26 resonances. Because the rigidity of the stllt legs 10~, when loaded hori- ~ ;;
27 zontally5 is many times greater ehan the ri&idity of piston rod 104 as a 28 cantilever~ only the stllt legs provide significant resistance to hori- ;~
~9 zontal loading. In doing so, the horizontal loading appears primarily as `~
t~nsion or compression in the stilt legs 108. The stilt legs provide _g_ ~-~

L13~Z~9 1 greater strength when loaded in ei~her compression or tension than when 2 subjected to bending forces. The use of inclined stilt legs is superior 3 because of the efficient structural use of its members. The efficiency of 4 the lnclined stilt legs permits the stilt leg dlmensions to be reduced below those which would be neces~ary if previously known stilt leg con 6 figuration (i.e., vertical legs) were employed. It will be seen, there-7 fore, that the unique stllt leg configuration further contributes to mini-3 mi~ation of the baseplate weight. The baseplate weight is reduced even 9 further by the elimination of the lower cross member 9 commonly used in ~`
lO previous vibrators. ~ `
11 For the purpose of imparting vibratory ~ovement to the piston 12 103, there is provided a manifold and servo valve member 19. The servo 13 valve follows the electrical control signal fed to it through conducting 14 leads l9a. Hydraulic actuatlng power for the servo valve is supplied through line 43 from an external hydraulic power source, including pump 42 16 and reservoir 48 (see FIGURE 2). The servo valve controls the flow of `;
17 hydraulic fluid into and from cylinder 101 through port means 64 and 66 -~
18 within the walls of cylinder housing lOla above and below piston 103 to 19 generate piston motion correspondlng to the electrical input control sig-nal. Hydraulic supply mechanisms are well kl~own in the art and need not be 21 discussed in greater detail here. One such mechanism is disclosed in U. S.
22 Patent No. 3,929,206.
23 The hydraulic vibrator generates a force against the ground by 24 pushlng against a reaction mass comprising the mass of cylinder housing 25 lOla, plus additlonal mass affixed to the cylinder housing. This addition- `
26 al mass includes maniEold and servo valve member 19, which in the preferred 27 embodiment is mounted on the forward portion of the vibrator~ The addi-28 tional mass further includes rear balance weigh~ lOlb and side weights lOlc `
29 and lOld. Thus, it will be seen that the reaction mass is a "multi-piece bolt together" mass consisting of a center section lOla which houses the -~;" ', --10-- .`-~ 31~3~

l actuator, two slde weights lOlc and lOld, a rear balance weight lOlb, and ;`
2 the manifold and servo valve member 19. The mass pieces are shaped so that 3 the stil~ legs pass through but do not touch the mass shapes.
~; The weight of the hydraulic cylinder housing and mass attached thereto is decoupled from the baseplate mass by air sprlng 37 which is attached to the top of frame member 106. Two vertically ex~ending membe~s, 7 130a and 130b, connect the top of the rea~tion mass to a frame menlber 134 ;~ ~
~ affixed to the top of the air spring 37. Air spring 37 is an isolation ~ -g spring and a].so sets the average position of the hydraulic piston 103 in the center of the cylinder to ensure more linear operation of the hydraulic ll vibrator. Air spring 37 is inflated ~o a desired pressure through a con-12 ventional fill valve shot~n as valve 140. l~o arcuate sections 136a and 136b 13 ~the latter not shown) are affixed to the bottom of frame member 134 on 14 opposite sides thereof, and a layer of elastic material 138 is attached to the lower edges thereof. This elastic layer serves as a buffer when the 16 up-stroke of the hydraulic~piston is too large. A comparable structure 17 (not shown) functions as a buffer when the down-stroke is too large. ;-~
18 As is common practice in the art~ the body of the vibrator is 19 located between the~Erame members of the truck. In the usual design of hydraulic vibrators, the baseplate extends outwardl.y from the vibrator, and 21 vertical guide rods and hydraulic lift cylinders extend from the truck 22 frame to a "footpiece" above outwardly ex~ending portions o~ the baseplate.23 The weight of the vibrator transport truck is applied to the baseplate 24 through the "footpiece" and spring lsolation means to assist in holding thebaseplate on the ground. In the present vibrator, in addition to milling 26 out portions of the baseplate, the dimensions of the baseplate ha~e also 27 been reduced in order to reduce the moving mass. Because of the reduced 28 size, the baseplate does not extend from beneath the vibratory body suffi-29 ciently to permit applying the truck weight to the baseplate in the con-ventional manner. A tmique one-piece hold~down plate 50 permits the use of -11- ~.

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l the small baseplate. The welght of the truck is transferred to the hold-2 down plate hydraulic lift cylinders 5 and 7 (see FIGURES 1 and 4). The 3 hold-down plate extends beneath the reaction mass and rests upon four air 4 bags 33a, 33b, 33c, and 33d (the latter two not shown in FIGURE 1), which are affixed between the hold-down plate and the baseplate. The air bags 6 may preferably be spaced at regular in~ervals around the basepl2te to 7 couple the weight of the truck to the baseplate evenly. The vibrator 8 piston rod extends through the center portion of the hold-down plate which 9 has a cut out section for that purpose. In addition to permitting the use of a small baseplate, the hold-down plate also provides a means for dis-11 tributing the air springs 50 as to couple the weight of the transport truck 12 to the baseplate in a more uniform manner about the surface of the base-13 plate. In the preferred embodiment, air bags 33a~ 33b, 33c, and 33d are 14 pneumatically isolated. It is possible, however, for the air bags to be -~
pneumatically coupled witho~lt departing from the spirit and scope of the 16 invention.
Hydraulic lift cylinders 5 and 7 (see FIGURES l and 4) control 18 the vertical position of the vibrator relative to the truck. The cylinder 19 housings of lift cylinders 5 and 7 are affixed to the truck frame and the 20 piston rods thereof are affixed to the hold-down plate. When hydraulic ;~
21 fluid is pumped into the upper portion of the lift cylinders 5 and 7, the 22 pistons are forced down relative to the cylinders and ~he vibrator is `~
23 lowered to the ground. After the baseplate is lowered to the earth's 24 surface, if additional hydraulic fluid is pumped into the upper portions of ~ ~
25 lift cylinders 5 and 7, the ~ruck will be lifted of the ground and its i 26 weight will bear on the hold-down plate. The air springs which inter~
27 connect the hold-down plate wlth the baseplate transmit the weight of the 28 truck to the baseplate. The truck is lowered back to the ground and the 29 baseplate lifted off the ground by pumping hydraulic fluid into the lower ~ 3~

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1 portions of the lift cylinders 5 and 7. As the baseplate is lifted off the 2 ground, it is suspended from the hold-down plate by means of a plurality of 3 chains 8. Guide rods 6a and 6b (see FIGURES 1 and 4) slide through the -~
4 cylindrical bores of guide frames 9a and 9b (9a not shown) which are ;

5 rigidly affi~ed to oppoæite sides of the transport truck. These guide rods ;~

6 are normally interconnected so that they move up and down in unison.

7 Techniques for performing this function are well known in the art and need ~-~

8 not be discussed here, one such technique being shown in the aformentioned ~

9 U. S. Patent No. 3,929,206. ~-Because air springs 33a, 33b, 33c, and 33d have little resistance 11 to lateral stress, a linkage mechanism is used to maintain the baseplate in 12 vertical alignment with the vibrator and to apply the weight of the truck 13 substantially to the center of the baseplate. This horizontal stabiliza~
14 tion is required to not interfere with or detract from, to any appreciable 15 extent, the desired vertical motion of the baseplate. Further, the hori- -16 zontal stabili~atlon mechanism should impose no appreciable horizontal 17 motions to the truck. It is known in the art to use for this purpose, a 18 multiplicity of radius rods, one end of each radius rod being pivotally 19 attached to the baseplate, and tbe other end being plvotally attached to a 20 part of the llft ~echanism of the truck. ;~
21 'rhis radlus rod type of stabilization system provides good hori-22 zontal control between the baseplate and the hold-down plate of the vehicle 23 lift system. The radius rod system, however, does impose undesirable ~
24 horizontal motion to the hold-down plate as the baseplate moves vertically. ~-The undesirable horizontal motions hecome more severe as the radius rod 26 length i9 decreased, as it would have to be if this type of stabilization 27 system were used on the BBV. It i5 common practice in the design of vi-28 brators to use palrs of radius rods mounted so that they are in opposition.
29 The tendency for horizontal displacement as the vlbrator moves vertical]y is taken up by rubber bushings in the eye ends of the radius rods. Deflec-31 tion of the rubber bushings, however, takes energy from the vibrating 32 baseplate.

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__ . . 1 ~ ,: ,' , , , ., ' . ' . , 1 Wlth reference to FIGURES 1 and 5, a linkage mechanism designated 2 by numeral 150 is ~Ised to maintain the baseplate of the BB~ in vertical 3 alignment with the truck. The linkage mechanism is comprised of equal 4 length rods 152a and 152b and rotating center link 154. An end of each equal length rod is connected to a mounting assembly 156 rigidly affixed to 6 the corners of the baseplate. The other end of each equal length rod is 7 connected to center link 154. Center link 154 is connected in the center 8 thereof in a rota~ing manner to plate 158 wh-Lch extends downwardly from the9 hold-down plate in substantially vertical alignment with the edge of the hold-down plate. Center link 154 rotates freely about this center con-11 nection and the two equal length rods rotate freely about a connection to 12 opposing ends of center link 154. The ends of the equal length rods con-13 nected to mounting assembly 156 are also rotationally free. One of these 14 linkage mechanisms 154 is affixed to each side of the vibrator baseplate.
This mechanism permits the baseplate to move up and down relative to the 16 hold-down plate while maintaining vertical alignment. When the baseplate 17 moves down in relation to the hold-down plate the center link 154 will 18 rotate in a counterclockwlse direction. When the baseplate moves up rela-19 tive to the hold-down plate, center link 154 rotates in a clockwise direc-20 tion. Ln a preferred embodiment, the ends of rods 152a and 152b may be `
21 connected by a ball joint to the mounting assembly connected at the corners22 of the baseplate, to permit an additional degree o freedom. This Iinkage :
23 mechanism permits the baseplate to tilt relative to the hGld-down plate.
24 Therefore, if the vibrator is on ground which is not in parallel alignment wi~h the plane of the truck, the baseplate can tilt so as to rest evenly on 26 the ground.
27 In the preferred embodiment, the center lin~ and its pivot are 28 attached to the hold-down plate, whlle the extremities of rods 152a and 29 152b are pivotally attached to supports on the baseplate. Alternatively, 30 the center link and pivot may be attached to the baseplate, while the ``

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1 extremities of the rods can be pivotally attached to supports on the ~lold-2 down plate. '~le arrangement of the preferred embodiment, however, has a 3 distinct advantage when the vibrator is operating on sIoping ground. With 4 reference to FIGURE 6a, which illustrates the arrangement of the preferred embodiment, the baseplate is driven by the vibrator along a line p~rpen-6 dicular to the baseplate and ground. The baseplate is also guided along 7 this same line by the constraint imposed by the linkage. FIGURE 6b illus-8 trates the arrangement of the alternative embodiment wherein the baseplate 9 motion is again along a line perpendicular to the baseplate and the ground, ~' while the linkage tends to constrain the motion to a vertical line. Thus, 11 the stabilizing linkage in this case will impose horizontal forces between ` ' 12 the baseplate and ~he hold-down plate.
13 In the preferred embodiment, stabilizing rods 152a and 152b have 14 equal length and center link 154 is symmetrically dlsposed about its plvot point. It is also within the contemplation of the invention to use sta-16 bilizing rods of unequal length and a non-symmetrical center link.
17 In the preferred embodimentl the length of each o~ stabilizing 18 rods 152a and 152b is 19 inches, while the distance between the points at 1~ which the stabili~ing rods are coupled to center link 154 is 5.625 inches. ~`
20 As an example of the effectiveness of the stabilizing means, if the base- ;
21 plate moves vertically with respec~t to the hold-down plate by a distance of 22 2 inches, there will be a relative horizontal displacement between the 23 baseplate and hold-down plate of 0.00021 inches. 'If a radius rod suspen~
24 sion were used in this case (with a rod length of 38 inches), for a ver~
tical displacement of 2 inches there would result a horizontal displacement 26 of 0.0527 inches. Thus, the stabilizing linkage employed in the BB~ re-27 duces the horizontal displacement by a factor of 250.
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28 From the foregoing, it will be seen that the unique stabilizing 29 system provides several important advantages. Vertical translation of the baseplate results in negligible horizontal motion imparted to t'he support-31 ing truc'k by the stabili~ation system. Further, the space required for the ~;

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1 stabilization system is reduced. Finally, the energy absorbed by the 2 stabilization system is reduced, thereby increasing the net seismic energy 3 into the ground.
4 FIGURE 7a is a sectional view of the cylinder area of the re-action mass from a prior art vibrator. FIGURE 7b is a sectif~nal view of 6 the corresponding portion of an embodiment of the present invention. All ; ;
7 vibrators are provided with an over-travel limit system which serves to 8 prevent the piston from traveling beyond its nominal stroke to the point 9 where the piston impacts an end of the cylinder. Most vibrator actuators .
have springs or externally mounted hydraulic shock absorbers as part of the 11 over-travel limit system. That type of over-travel limit system has 12 several features which render it undesirable for use in the present vibra-13 tor. The first problem relates to the effective oil volume ln the cylinder 14 of the actuator. A frequency is reached at which the volume of the oil within the cylinder acting as a sprlng resonates in conjunction with the 16 load mass. This oil column resfonance places an upper li.mit on the range of 17 frequencies over which the~vibratfor can operate. The frequency of oil :

lfB column resonance varies inversely with the square root of effective volume 19 of oil within the cylinder. ~ccordingly, it will be seen that iD the BBV

it is essential to minimize this effective oil volwlle.

21 As the piston moves from the center of the cylinder to the limit 22 of its stroke, it sweeps a~volume of oil from the piston equal to the 23 product of the piston area and its length of stroke. If the piston travel, 2f~ however, exceeds its nominal stroke, it will sweep an additional volume of oil equal to the product of the piston area and the length of over travel.

26 Thus, ~his additional oil volume, for which no benefit is received, is ` ^

27 proportional to the distance that the piston over travels before the over-28 travel limit system stops it. The amount of over travel with prior art 29 externally mounted limit systems is large and results in an effective cylinder oil volume that is undesirable for the present invention.

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1 Additionally, the use of external shock absorbers reduces the re~
2 liability of an over-travel limit system. For example, if the plunger 3 fails to reset due to a broken return spring or if not enough time has ; ~;~
~ elapsed for a normal resetting before the over travel condition repeats, no shock absorber action will occur. In an embodiment of the present inven-6 tion, there is pro~ided an internal over-travel limit system which is reset 7 if the actuator is in the working stroke. While an internal aver-travel 8 limi~ system is known in the prior art, the internal limit system of the 9 present invention has several features which render it more advantageous for use in a high frequency vibrator.
11 The essential features of the prior art internal limit system are 12 illustrated in FIGURE 7a. There is shown in sectional view a portion of a ` ~;
13 reaction mass 200 including the cylinder region of the reaction mass. An 14 axial hollowed out portion of circular cross sect-on extends through the reaction mass. This hollowed out portion is lined over part of its length 16 by bronze bushings 202 and 204 which serve to support in sliding relation- `~
17 ship, the pis~on rod 206~of a double ended piston. The cyllnder area of 18 reaction mass 200 is lined by a sleeve 208 of a metal such as cast iron.
19 Piston 210, including a plurality of piston rings, is reciprocably located within the confines of the cylinder. Clearance is provided between the 21 walls of the piston and the inner surface of sleeve 208 so that only the 22 piston rings are in contact with sleeve 208. Ports 212 and 214 communicate 23 with the manifold ~not shown) so as to admit oil under high pressure al-24 ternately to opposite sides of the piston. Port 212 opens into an annular passage 216 which e~tends around the outer circumference of bushing 202.
26 Passage 216 in turn communicates with a plurality of holes 218 located in 27 and about the circumference of bushing 202. Thus, it will be seen that 28 high pressure oil from the manifold is admitted through port 212, passage 29 216 and holes 218 into a portion of the cylincler formed by the inner surface ; -"

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1 of bushing 202 and a narrowed portion of piston rod 206. The oil so ad-2 mitted may flow into the main body of the cylinder so as to generate a 3 force acting against one side of the piston. In a similar manner, oil from 4 the manifold flows through port 214, channel 220 and holes 222 to the opposite end of the cylinder.
6 The internal braking action provided by the illustrated structure ;
7 may be appreciated from tbe following brief operational discussion. Let it 8 be assumed that oil is admitted under high pressure through port 214 to the ,~
9 right side of the cylinder. This causes the piston 210 and piston rod 206

10 to move to the left and oil from ~he left side of the cylinder is exilausted `

11 through port 212 to a reservoir of low pressure oil. This continues until

12 shoulder 224 of the piston rod reaches and begins to enter bushing 202. At

13 this point in time, a volume of oil is trapped in region 226 of the cylin~

14 der. This trapped volume of oil imparts a braking action to the piston and

15 prevents it from impacting the end of bushing 202. The piston braking is ~ `

16 accomplished by the trapped volume whicn escapes slowly through the small

17 clearance provided between the portlon of piston rod 206 which i5 not

18 narrowed and the inner surface of bushing 202. Because of the high pressure

19 occurring in region 226 during braking action, it is necessary to provide an 0-ring 228 for preventing the escape of oil between bushing 202 and 21 reaction mass 200. A corresponding O-ring 230 is provided at the other end 22 of the cylinder. In tb~e~structure illustrated, it will be noted that the 23 piston and plston rod are unsupported over the entire region between 24 shoulder 232 and shoulder 2340 In the prior art vibrator illustrated, this 25 is a length of approximately 16.5 inches. `

26 The internal hydraulic braking action provided by an embodiment 27 of the present invention may be illustrated with the aid of FIGURE 7b which 28 shows a sectional view of a portion of the reaction mass lOla. Reaction -;

29 mass lOla is provided with an axial bore extending through its entire length.

.:
' 1 The bore is lined at its ends by bushings 252 and 254 which 2 provide bearlng surfaces for the piston rod 104 of a double ended piston 3 103. In the preferred embodiment bushings 252 and 254 are made of bronze.
4 Other materials, however, may be used for the bushings. One such suitable ~`
material is a polymide manufactured under the trade mark VESPEL by Dupont.
6 Rush~ngs 252 and 254 also serve to support piston 103 in the areas indi-7 cated by reference designators 260 and 262. In the region of the piston `~
8 rings, the surface of the bore is lined by a sleeve 264 of a metal such as 9 cast iron. j ~ -Ports 64 and 66 communicate with the manifold (not shown) to 11 serve as input and exhaust ports for oil entering and leaving the cylinder.
l2 Port 64 opens into an annular passage 270 which extends around the outer 13 circumference of bushing 252. Passage 270 in turn communicates with a 14 plurality of slots 272 formed in bushing 252 whereby oil is admitted to and exhausted from one side of the cylinder. In a similar manner, port 66 16 cooperates with annular passage 274 and slots 276 to provide a path for the 17 oil to the opposite end of the cylinder. -18 During operation of the vlbrator 9 as high pressure oil is ad-19 mitted through port 66 so as to exert a force against the piston driving it to the leftS oil is forced out through port 64 to a low pressure reservoir.
21 During normal operation, prior to the tinle when piston 103 passes the `~
22 plurality of ports 272, the flow of oil will be reversed so as to admit -~
23 high pressure oil to port 64 and allo~ oil to leave the cyllnder through 24 port 66 to the low pressure reservoir. Under abnormal conditions, however, the piston may exceed its designated stroke and travel sufficiently far to 26 the left to close off slots 27Z, thereby trapping a volume of oil in the 27 end of the cylinder. This trapped oil is bled back to the slots through 28 the radial clearance between piston 103 and bushing 252. The concentricity --19~
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1 and radial clearances are selected to obtain a desired shock absorber ;
2 action. The equation used to predict the shock absorber response is:

3 F = 6u~(r2 - ro2)2LL' RC (1 32E ) ;~

4 where FD = Retarding force, 6 u = Oil viscosity, 7 L = Length o engagement 8 L' = Relative veloclty between the rod and g rod bushing, ro = Small rod diameter, 11 R = Rod bushing cavity radius, 12 ~ r = Piston radius (plunger), 13 C = R - r = radial clearance, 14 e = eccentricity, piston relative to cavity, and 1~ = C~

16 The internal shock absorber illustrated in FIGURE 7b is superior 17 to the prior art arrangement of FIGURE 7a in several respects. First, 18 piston 103 is always engaged in bushings 252 and 254 at regions 260 and 262 ~ ;
19 respectively. Accordingly, there is no mechanical "plunger" insertion into the bushing. This enhances the reliability of the mechanism.
21 Secondly, since mechanical support for the piston is provided by ~
22 bushings 252 and 254 in regions 260 and 262, respectively, the longest ~ -23 unsupported length of the actuator rod structure is that portion of ths 24 piston that is ~nclosed within liner 264. In the preferred embodiment, this unsupported portion of the piston sxtends only oYer a length of 9.2 26 inches. By reducing the unsupported length of the piston, the stresses

-20-,', ' 1 exerted on the actuator rod are relatively reduced. As will be discussed 2 below ln connection with FIGURE 8, this permits the piston rod assembly to 3 be hollow, thereby further reducing the weight of the baseplate and asso- -4 ciated elements.
5 When the over travel condltion occurs, in the preferred embodi- i 6 ment the oil ls trapped between the piston 103, the piston rod 104, and 7 either of bushings 252 and 254. As a result, it is not necessary to pro-8 vide O-rings corresponding to O-rings 228 and 230 in FIGURE 7a. These 0-9 rings are required in the prior art structure since, there, the braking oil volume is trapped ahead of bushings 202 and 204. Eliminatlon of the 0-ring 11 reduces the actuator cost. Further, it will be noted that piston rod 104 12 does not require the additlonal machining operations required to produce 13 the reduced diameter section of piston rod 206 in ~he prior art structure.
14 Finally, in the preferred embodiment, there i5 no oil volume corresponding to that volume of oil located between the reduced diameter section of 16 piston rod 206 and bushing:s 202 and 204 in the prior art~structure. As -~ -17 mentioned previously, this reduced oil volume is advantageous in high 18 frequency operation of the vibrator.
1~ FIGURE 8 is a sectional view showing the configuration of the actuator rod 280. The rod is hollow so as to reduce the baseplate weight

21 of the vibrator. The tapered bore of the rod i8 specifically designed such

22 that the stresses resulting from forces applied transverse to actuator rod `

23 280 are approximately constant as a ftmction of distance along the rod. As

24 a result9 no portion of the rod is "over-designed" (and correspondingly over-weight) relative to another portion of the rod.
26 The diameter of the sleeve in which the piStOII 103 runs, in the 27 preEerred embodiment, is nine inches, while the diameter of the piston rod 28 is seven inches. As a result the effective plston area at either end of ~: .

a` ~L 3~ ;i9 l the piston is 25.13 square inches. Hydraulic fluid is supplied to the BBV
2 at a pressure of 3000 psi. Accordingly, it will be seen that the peak 3 force acting on the piston of the RBV is 75,390 pounds, far in e~cess of 4 that used in other vibrators.

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There has been disclosed a new seismic vibrator, suitable for 6 operation over a broad band of frequencies. Whereas the preferred embodi-7 ment of the invention has been disclosed, there may be suggested to those : : ..
3 skilled in the art certain minor modifications which do not depart from the 9 spirit and scope of the invention as set forth ln the appended clalms.

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Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A vibratory seismic energy source adapted to be transported on a vehicle, comprising:
a reaction mass having a vertically disposed power piston and cylinder assembly formed therein, a baseplate for coupling seismic energy to the ground;
a piston rod extending downward from the piston and connected at its lower end to said baseplate;
a pair of substantially vertically disposed support columns comprising piston-cylinder assemblies adapted to support the weight of the transport vehicle thereon, one of said support columns being positioned to each side of said reaction mass;
a rigid plate interposed between said reaction mass and said baseplate and adapted to permit motion of said piston rod in relation to said rigid plate, said rigid plate extending laterally outward to each side of said reaction mass to enable said support columns to be attached to said rigid plate, and elastic means for supporting said rigid plate from said baseplate in resilient, movable relation therewith.

2. The apparatus of claim 1 wherein said elastic means comprises a plurality of air springs positioned between said rigid plate and said baseplate.

3. The apparatus of claim 1 wherein said baseplate does not extend laterally beneath said substantially vertically disposed support columns.

4. The apparatus of claim 1 further including a plurality of con-necting means affixed to said rigid plate for connecting lateral stabilizing means which extend between said rigid plate and said baseplate to said rigid plate, one of said connecting means being connected substantially in the front center of said rigid plate and one of said connecting means being connected substantially at the rear center of said rigid plate.